Contrast agents are widely used to enhance magnetic resonance imaging (MRI) contrast. The administration of traditional MRI contrast agents, such as gadolinium (Gd) chelated to DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), are thought to achieve contrast by the paramagnetic relaxation effect of a metal-ion to shorten the bulk water relaxation time via rapid exchange of the metal ion's inner-sphere water molecules with bulk solvent.
An alternative class of MRI contrast agents rely on chemical exchange saturation transfer (CEST) to enhance MRI contrast. For instance, the water exchange rates for water molecules bound to certain extrinsic paramagnetic metal ion-macrocyclic complexes incorporating the tetraazacyclododecane macrocyclic ring are sufficiently slow that a separate bound water magnetic resonance signal, substantially up-field or downfield (e.g., about ±6 ppm or more) from the bulk water magnetic resonance signal, is observable in pure water as solvent. This highly shifted and slowly exchanging bound water molecule may be irradiated to produce magnetization transfer (MT) on bulk water and thereby serve as an effective contrast agent.
For both traditional MRI and CEST contrast agents, the residence lifetime (τM), is an important factor governing the effectiveness of a given contrast agent. The term τM as used herein refers to duration that inner-sphere water molecules associate with the paramagnetic metal ion-macrocyclic complexes. A water molecule that resides too long on the metal center of a traditional MRI contrast agent, for example, will occupy space that could otherwise be used to effect the paramagnetic relaxation of different water molecules, thereby reducing the overall effective paramagnetic relaxation of bulk water. On the other hand, if the water molecule does not reside on the paramagnetic metal ion-macrocyclic complexes for long enough, then effective paramagnetic relaxation also does not occur. It is generally believed therefore that traditional MRI contrast agents should optimally have a TM that is between about 10 and about 100 nanoseconds.
Contrary to the above-described traditional MRI contrast agents, it is desirable for CEST contrast agents to have a slow rate of water exchange. For example, some of the above-mentioned extrinsic paramagnetic metal ion-macrocyclic complexes have sufficiently slow water residence lifetimes (e.g., τM greater than about 1 microsecond) such that the highly shifted bound water magnetic resonance signal is observable at room temperature. This, in turn, facilitates the identification and irradiation of the highly shifted bound water magnetic resonance signal.
It has proven challenging, however, to design contrast agents having the tetraazacyclododecane ring structure incorporated therein so as to provide the desired τM value. At least part of the problem stems from the dynamic equilibrium that typically exists between different enantiomeric configurations of both the macrocyclic ring and the pendant arms coupled to the ring. The resident lifetime of a water molecule that is associated with the contrast agent can be substantially different for each of the enantiomeric configurations of the contrast agent. This, in turn, effects the suitability of the contrast agent for particular applications.
As an example, consider a contrast agent comprising a Gd(III)-DOTA complex. The macrocyclic ring of the Gd(III)-DOTA complex adopts a quadrangular (3333) conformation, with the torsion angle between the square defined by the nitrogen atoms, and a second square defined by coordinating oxygen atoms, defining the coordination geometry of the complex. A torsion angle of about 39° defines a capped square antiprismatic geometry, whereas a torsion angle of about 25° defines a capped twisted square antiprismatic geometry.
The conformation of each ethylene bridge in the macrocyclic ring is designated as λ or δ, according to the sign of the torsion angle. Thus, the conformation of the macrocyclic ring has either a (δδδδ) or (λλλλ) orientation. Similarly, a torsion angle between the carboxylic groups of the acetate pendant arm, the metal ion, and the ring nitrogen that the pendant arm is attached to, can have either a positive or negative sign, designated as Δ and Λ, respectively. Thus, the pendant arms can have either a Δ or Λ orientation.
Consequently, the Gd(III)-DOTA complex has four enantiomeric configurations that are in dynamic equilibrium with each other. The four stereoisomeric coordination geometries are summarized as: Δ(λλλλ), Λ(δδδδ), Δ(δδδδ) and Λ(λλλλ). When the orientations of the pendant arms and macrocyclic ring are identical (e.g., Δ(δδδδ), and Λ(λλλλ), then the Gd(III)-DOTA complex adopts the capped twisted square antiprismatic geometry. But when the orientations of the pendant arms and macrocyclic ring are opposite (e.g., Δ(λλλλ), and Λ(δδδδ), then the Gd(III)-DOTA complex adopts the capped square antiprismatic geometry.
It is generally believed that the measured τM value for the inner-sphere water molecules (about 244 nanoseconds) associated with the Gd(III)-DOTA complex is actually a weighted average of the resident lifetimes for each of these four stereoisomeric coordination geometries. The relatively long τM makes Gd-DOTA undesirable in the design of high relaxivity contrast media (e.g., a relaxivity at 298° C., r1 298, of at least about 50 mM−1s−1) Moreover, it has been suggested that the resident lifetime of inner-sphere water molecules associated with the Gd(III)-DOTA complex is substantially different (e.g., about one or two orders of magnitude) for the two geometries.
Accordingly, what is needed is a contrast agent whose stereoisomeric coordination geometries can be selected among the available isomers, and thereby have the desired τM value for particular magnetic resonance imaging applications, thus avoiding problems encountered with previous contrast agents.